1. Field of the Invention
The present invention relates to a thin film semiconductor device, display, and method for manufacturing a thin film semiconductor device, and particularly to a thin film semiconductor device preferably used as a thin film transistor that employs a polycrystalline silicon film arising from crystallization by laser annealing, display employing the same, and method for manufacturing a thin film semiconductor device.
2. Description of the Related Art
In a flat-panel display such as a liquid crystal display and organic EL display, thin film transistors (TFTs) are provided as elements for driving pixel electrodes. Of the TFTs, a polycrystalline silicon TFT, in which a polycrystalline silicon film is used as a semiconductor thin film serving as an active region, has a higher carrier mobility compared with an amorphous silicon TFT, and is suitable as a switching element for a display that is necessary to have higher performance.
For manufacturing of the polycrystalline silicon TFTs, a low-temperature process has been being developed in order to realize formation of the polycrystalline silicon TFTs not over a quartz substrate but over a low-cost glass or plastic substrate. In this low-temperature process, a method of turning amorphous silicon deposited over a substrate into polycrystalline silicon by laser annealing is carried out.
However, a polycrystalline silicon film crystallized by laser annealing includes a large number of crystalline defects, and hence electrons moving in the film are easily trapped by the crystalline defects. Therefore, it is not preferable that a polycrystalline silicon film be used as it is for the active region of a thin film transistor. To address this, in some countermeasures, an insulating film containing a large amount of hydrogen ions is formed on a polycrystalline silicon film arising from polycrystallization by laser annealing, and the polycrystalline silicon film is subjected to annealing treatment under a nitrogen atmosphere together with the insulating film, to thereby bury the hydrogen ions in crystalline defects in the polycrystalline silicon film. As the insulating film containing a large amount of hydrogen ions, a silicon nitride film is generally known. However, it is known that transistor characteristics are deteriorated when this insulating film is deposited by using only a silicon nitride layer. Therefore, typically the insulating film is formed in such a way that the silicon nitride film is interposed between silicon oxide films (refer to e.g. Japanese Patent Laid-open No. 11-111989).
Furthermore, a configuration has also been proposed in which a polycrystalline silicon film is interposed between upper and lower silicon nitride films. In this configuration, hydrogen is supplied from one silicon nitride film to the polycrystalline silicon film, and the other silicon nitride film suppresses diffusion of coupled hydrogen (refer to e.g. Japanese Patent Laid-open No. 2001-125142).
As another problem, the polycrystalline silicon film crystallized by laser annealing is susceptible to the influence of contamination and so on because it includes a large number of crystalline defects, and thus element functions are deteriorated. To address this problem, a configuration has also been proposed in which silicon nitride films having a blocking function against mobile ions (contamination) are formed both over and under a TFT (refer to e.g. Japanese Patent Laid-open No. 2004-7004).